T-Mobile’s network modernization in Las Vegas is now complete, which means the carrier can now support mobile broadband speeds on the iPhone. T-Mobile CTO Neville Ray announced at GigaOM’s Mobilize conference on Friday that T-Mobile will begin marketing its “4G” HSPA+ service to unlocked iPhone users in Vegas on Monday.

T-Mobile has been overhauling its network, realigning them with the 3G bands supported on most North American GSM phones, while simultaneously clearing frequencies for its future LTE network. T-Mobile has already revealed that it has converted over thousands of cell sites in New York City, Seattle and Las Vegas and many of T-Mobile’s current customers using unlocked iPhones have already encountered big boosts in speeds on a site by site basis.

But Ray said Friday that the spectrum refarming process is now 75 percent complete in Vegas, providing the consistency of coverage necessary to let start marketing HSPA+ to potential iPhone users. Until now, any iPhone on T-Mobile’s network was limited to 2G EDGE speeds.

Older generations of the iPhone won’t be able to access the full bandwidth of T-Mobile’s dual-carrier HSPA+ network, which can support theoretical speeds of 42 Mbps — though they will still get 3G connectivity — but anyone willing to shell out the dough for an unlocked iPhone 5 would be able to access the fastest HSPA network in the country. LTE support on T-Mobile, however, won’t be available until next year when its network overhaul is complete.

As for the rest of the country, Ray said its quickly turning up the new HSPA+ network at PCS in cities nationwide. He wouldn’t reveal any specifics on when the process will be finished, but he has said that a “material” portion of T-Mo’s footprint will be iPhone ready by year-end.

Check out the rest of our Mobilize 2012 coverage here, and the live stream can be found here.